FOURIER TRANSFORM BASED ECE SYSTEMS FOR REAL TIME TEARING MODE CONTROL IN TOKAMAKS

The development of variable-resolution Electron Cyclotron Emission (ECE) diagnostics for Tokamaks based on the digitization and subsequent Fast Fourier Transformation (FFT) of the intermediate frequency signal [1] has enabled the exploration of the space-time domain of ECE diagnosed phenomena. This development depends on advances made in fast multi -Gigahertz sampling ADCs that have recently become available. One important application is the feedback control of neo-classical tearing-modes (NTMs) in Tokamaks by Electron Cyclotron Heating and Current Drive (ECH&CD) [2, 3 and 4]. This requires both a detection of the full mode content in the plasma and an accurate radial localization and poloidal phase of the magnetic islands. An FT ECE system has the possibility to optimize time and frequency resolution (spatial resolution). It could observe the plasma cross-section and then zoom-in on the detailed structure of the island. The scrutiny of the plasma phenomena at variable temporal and spatial scales could be extended by means of wavelet analysis.

A proof of principle on TEXTOR employing an FFT-ECE [1] system in-line with the ECH&CD system [5] enabled detailed observations of scattering of the high power mm-waves on rotating tearing modes [6]. The observed scattering is found to be in phase with the NTM island rotation. The medium and long-term objective is to develop FFT-ECE for NTM control on AUG and ITER.